1. Field of the Invention
The present invention concerns an alarm or warning device using, as sensing means, a low voltage D.C. biased shielded coaxial cable. This cable is connected to an electronic circuit which processes the voltage variations at the cable ends and energizes alarm or warning means as predetermined conditions take place.
The alarm or warning device of the present invention is used with particular advantages for the control of large areas and, generally, in alarm systems against intrusions and thefts in real and personal property, as well as in the medical field for the remote control and warning of the life functions of a patient. It has interesting and innovating characteristics both for what concerns its installation, which is practical and economic, and for what concerns its working, which is reliable due to the impossibility of eluding or impairing the device, and finally for what concerns its useful life.
2. Description of the Prior Art
Alarm devices using, as sensing means, electric circuits working as capacitors, have been known for a long time. Such devices detect the variations in the electric characteristics of said circuits (voltage or current) when a load weighs thereon, which serves to modify--though imperceptibly--their physical structure and, consequently, their electrical capacitance.
Such known sensing means normally consist of composite structure carpets, the conductive elements of which are connected to an electronic monitoring circuit. Carpets thus formed can simply be laid on the ground, or underground, and are adapted--as seen heretofore--to detect the loads weighing thereon.
Though, on one hand, such known sensing means have represented a positive improvement over the ones used in previous alarm systems--as they can be totally hidden and are unlikely to undergo false alarms--on the other hand they involve different drawbacks which have, up to date, limited their field of application. In fact, in the first place, such carpets involve high production costs, which make their use uneconomical when having to control fairly large areas. Moreover, the underground installation of carpet sensing means is particularly delicate and costly and, furthermore, such carpets can evidence in the long run problems of moisture tightness along their edges, which can irreparably compromise their functionality, forcing the use into costly replacements.
The most serious drawback derives however from the fact that the aforedescribed carpets have to be positioned in strategic points, where it is presumed that the intrusion having to be prevented is more likely to occur; whereby, they become quite useless, or else involve prohibitive costs, when these strategic points are too many (buildings with a lot of possible entries) or do not even exist (for instance, the fence of a factory, each point of which can be a possible point of intrusion).
Finally, for what concerns personal property, such carpet sensing means are fit only for movables of small size.